1. Field of the Invention
This invention relates generally to web transporting apparatus, and more specifically to a shuttle mechanism for accurately, intermittently advancing a stretchable web containing spaced apart, encapsulated articles.
2. Description of the Prior Art
Shuttle mechanisms for intermittently advancing a web a normal distance or pitch are generally well known in the art. For example, motion picture projectors, motion picture cameras and photographic printers all utilize a shuttle claw mechanism in which a claw enters a film perforation, is advanced a film pitch equal to the pitch between succeeding perforations in the film for advancing the film a film frame, is retracted from the film and returned to its initial position in which the claw enters the succeeding film perforation. However, in such applications, the film is relatively inelastic and unstretchable. Consequently, the film is advanced a correct or normal web pitch for each cycle of operation of the shuttle claw mechanism. A problem is presented, however, in those applications in which a web is intermittently advanced which is partially elastic or stretchable, such as occurs in the manufacture of dental X-ray packets. In such an application, several webs of various types of materials are combined to make a completed web containing spaced apart, encapsulated dental packets completely surrounded by sealed areas. The shuttle mechanism must intermittently advance the web, and for each advance must accurately align the dental packet with a rectangularly formed punch so that a uniform sealed area is present around the entire periphery of the packet when it is punched out of the web. Since the web is formed from a stretchable material such as polyvinyl chloride and is advanced against a variable web supply tension, it is impossible, due to the web stretching that occurs, for the shuttle mechanism to intermittently, accurately advance the dental web a normal pitch. Consequently, misalignment of the dental packet with the punch results causing the punch to cut through or seriously weaken one or more of the sealed edges of the dental packet.